Offenhauser makes two different manifolds for the AMC 6 cylinder engines. One is the Dual Port and the other is the “C” (Competition) series manifold. The Dual Port completely separates the primary portion of the carburetor from the secondary portion of the carburetor from the carburetor mounting flange to the cylinder head. Further, the primary runner is smaller in overall area. This increases the air/fuel velocity in the manifold at lower engine RPM which, in return, fills up the cylinder better. This increases low end torque. In today’s terminology the Dual Port is variable induction for carbureted engines.
A 258 CID engine does not need a 500 CFM carburetor until the engine reaches approximately 7,000 RPM (assuming 100% volumetric efficiency). That being said, the Dual Port manifold is designed to operate best from idle to 5,000 RPM. A 500 CFM carburetor is the largest CFM carburetor Offenhauser recommends for use on a 258 with the Dual Port manifold.
The “C” (stands for Competition) series manifold is a open plenum manifold. This allows for a higher volume of air/fuel mixture to move through the manifold. The C series manifold does not produce the same torque as the Dual port does at low RPM’s, but it does allow the engine to produce more horsepower in Mid and High RPM applications. Because of this, Offenhauser recommends, at a minimum, a 500 CFM carburetor on a 258 using a C Series intake manifold.